An LED consists of a semiconductor junction, which emits light due to a current flowing through the junction. Since the purpose of an LED is to emit light, it is often desirable for this light to be as constant as possible, both during operation of a device and also from unit to unit. Many designers of LED circuits use a constant current circuit for this purpose, because this gives a better regulated amount of light output than driving it with a voltage limited by a resistor.
However, the constant current drive still has a number of drawbacks. Among the chief of these is that, although the current through the LED is constant, the forward voltage of the junction is not. The light output of the LED is dependent on its input power, and this power depends on both the junction current and the forward voltage. Any variation of forward voltage thus directly results in variation in output light.
The variation in forward voltage in the LED has two main sources. One is the temperature of the junction. As the LED warms up, its forward voltage decreases, typically 2 to 4 mV/° C., or 0.06 to 0.11%/° C. While this seems small, LED temperatures in normal operation will typically range from 25° C. to at least 85° C., and over this temperature range, the variation in forward voltage can be as much as 6.7%. A variation of this size in light output, when combined with other factors, can be quite undesirable.
The other main source of variation in forward voltage in LEDs is manufacturing tolerance. A typical white LED may have a forward voltage specified to be between 2.8V and 4.0V. This variation translates directly to a variation in light output when using a constant current drive. As a consequence, LED manufacturers typically bin their parts, typically in 100 mV bins. This can reduce the variation to some 2.8%, but taken together, the two effects may still account for almost a 10% variation of light from unit to unit and from cold to hot.
One solution to this problem is to measure the forward voltage of the LED and provide a drive such that the product of this forward voltage and the drive current is constant. In practice, however, because the LEDs may not be ground-referenced, it becomes necessary to use expensive components to level shift the forward voltage signal to where it can be used by the control circuit.
Another partial solution is to measure the temperature of the LED, for example with a thermistor, and use the measurement as a feedback to the control circuit to adjust the drive current. While this concept works in some situations, it can be difficult to implement if the LEDs are not conveniently located. To measure the temperature requires two additional connections from the location of the LEDs for the thermistor, in addition to the two connections required to power the LEDs. Additionally, the control circuit must be configured to accept the input from the thermistor. If the signal is not acceptable, it must be conditioned with additional circuitry, or with a microcontroller. However, this method does not compensate for factory variations in forward voltage.